In the paper industry, the kraft process is valuable for paper production. The kraft process as taught by U.S. Pat. No. 3,245,635 to Liebling is generally described as being performed by first cooking the wood chips in digesters and then drawing off the spent chemicals for reuse. The resulting pulp fibers are then screened and then washed free in brownstock washers of a large amount of residual chemicals. These brownstock washers are a series of stages, typically vats, usually three or four in number which alternatively dilute the pulp with water and thicken it by picking it up on large rotary screens. In the conventional kraft paper making process the above-described brownstock washing step is performed in a stage or a series of stages (e.g., three or four stages) in which the water travels countercurrent to the pulp. Typically, there is one vessel, e.g., vat, employed per stage 10, 20, 30, 40 as shown by FIG. 1.
The upstream stages, e.g., stage 10, have high solids concentration, e.g., 10-20%, high pH, over 12 due to base, e.g., sodium hydroxide or potassium hydroxide, employed during delignification processing, and high temperature, about 190.degree. F. The downstream stages, e.g., stage 40, have low solids concentration, e.g., 0.1-0.3%, lower pH, ranging from about 8-12, e.g., 8-10, and lower temperature, about 120.degree. F. These stages all employ defoamers. However, because of the differing conditions, the prior art employed different defoamers in the different stages. Brownstock defoamer is employed in the earlier stages, e.g., stage 10, and screen room defoamer is employed in the later stages, e.g., stage 40. Screen room defoamer is similar to brownstock defoamer except that screen room defoamer includes emulsifier. To employ the same defoamer in all the stages would avoid optimum performance because in the first stage the high temperature, high solids content and high pH help to distribute the insoluble defoamer throughout the black liquor and brownstock in each stage. However, these factors are not present in the later stages, e.g., stage 40. The later stages are colder, have less solids and are less foamy than the earlier stages.
From the brownstock washer stage 40, the clean pulp travels to the screen room where it is again diluted with water and put through vibrating screens which now accept the completely delignified fibers and reject clumps of unpulped fibers, knots, and other foreign material. Rejected material is recycled to delignification. Foam problems are severe in the brownstock washer vats since the diluted pulp is subjected to violent agitation by the rotary screens. Foam problems are also severe in the screen room since the diluted pulp is subjected to violent agitation by the screens. The water removed from the pulp after the screening operation is referred to as the dilute black liquor and, for the sake of economy, is normally used as the dilution water for the third and fourth stage of the brownstock washers. The dilute black liquor is a foaming material, containing from about 0.001% to 0.1% by weight of solids and has a pH of about 12. The foaming of the dilute black liquor increases with the resin content of the wood used in this process. After processing in the screen room, the pulp is processed in a decker. If whitening the brownstock is desired, then, the brownstock pulp is sent to a bleach plant, which typically includes one or more bleaching units and one or more caustic treating units.
Defoamers are generally used in most alkaline pulp mills during the brownstock washing, screening and bleaching operations so that more efficient washing, screening and bleaching, respectively are accomplished.
Another kind of pulp washing apparatus is known as a horizontal belt washer. A typical such washer is shown on FIG. 2 and described in Handbook For Pulp & Paper Technologists, prepared under the direction of the Joint Textbook Committee of the Paper Industry, G. A. Smook, Author, M. J. Kocurek, ed., (1982), available from TAPPI, Technology Park/Atlanta, P.O. Box 105113 Atlanta, Ga. 30348.
Pulp slurry containing up to 3.5% pulp is distributed across a traveling endless "wire" 102 (FIG. 2). The pulp is dewatered, forming a mat of about 8% to 12% solids.
The washer device 100 includes wire 102 and suction boxes 110, 120, 130, 140, 150, 160. Water is showered on the wire 102 over the suction boxes.
This pulp is then washed in a series of displacement stages as the pulp travels from the headbox 104 to the couch roll 106. The wire 102 passes along roll 103, 105 and 106. The mat's consistency remains constant at 8% to 12% during these washing stages.
The cleanest wash water is added in the final shower over suction box 160 ahead of the couch roll 106. Then water is drained through the wire 102 with suction box 160, then sent to the previous shower over the previous suction box 150. The filtrate from the first shower over suction box 120 is finally sent to the evaporators. Filtrate drained ahead of the first shower is used for dilution of furnish in headbox 104.
Belt washers, thus, use one dilution/extraction stage, followed by several displacement stages. These are all termed "wash zones" for purposes of this specification. Theoretically, a large number of displacement stages can be fit along the wire 102. Horizontal belt washers are supposed to give high overall washing efficiencies at comparatively low dilution factors.
The wire 102 on the belt washer can be of three basic designs. It can be a grooved rubber belt, a woven plastic filament belt, or made of a thin sheet of solid stainless steel, which has been perforated and welded to become a continuous belt. Typically washer 100 includes a hood 200.
It would be desirable to provide one defoamer and one emulsifier that could each be stored in a respective tank at the paper manufacturing site and employed in all of the above-described staged pulp treating operations as appropriate.
Moreover, paper pulp plants have a need for flexibility. The treating chemicals at one plant are not necessarily effective at another plant. Sometimes this lack of effectiveness is due to different feedstock being processed at the different plants. Also, even at the same plant, changes in feedstock can lead to a need to change treating chemicals. It would be desirable to provide a process where the same chemicals could be used at a variety of plants and with a variety of feedstocks.